1. Field of the Invention
The present invention relates to a process for producing a light emitting device using a light emitting element having a film containing an organic compound which can give luminescence by receiving an electric field (the film being referred to as an “organic compound layer” hereinafter), an anode, and a cathode. The present invention relates particularly to a light emitting device using a light emitting element having a lower driving voltage and a longer life span than conventional light emitting elements. The light emitting device referred to in the present specification is an image display device or a light emitting device using a light emitting element. Additionally, a module wherein a connector, for example, an anisotropic conductive film (such as a flexible printed circuit (FPC) or a tape automated bonding (TAB) tape, or a tape carrier package (TCP)) is set up onto a light emitting element: a module wherein a printed wiring board is set to the tip of a TAB tape or a TCP; and a module wherein integrated circuits (IC) are directly mounted on a light emitting element in a chip on glass (COG) manner are included in examples of the light emitting device.
2. Description of the Related Art
A light emitting element is an element which emits light by receiving an electric field. It is said that the luminescence mechanism thereof is based on the following phenomenon: by applying a voltage to an organic compound layer sandwiched between electrodes, electrons injected from the cathode and holes injected from the anode are recombined in the light emitting center of the organic compound layer to form molecular excimers; and energy is radiated when the molecular excimers return to the ground state thereof.
The kinds of the molecular excimers which are made from the organic compound may be a singlet exciting state excimer or a triplet exciting state excimer. In the specification, luminescence (that is, light emission) may be luminescence based on either of the two, or luminescence based on the two.
In such a light emitting element, its organic compound layer is usually made of a thin film having a thickness below 1 μm. The light emitting element is a natural light type element, wherein the organic compound layer itself emits light. Therefore, backlight, which is used in conventional liquid crystal displays, is unnecessary. As a result, the light emitting element has a great advantage that it can be produced into a thin and light form.
The time from the injection of carries to the recombination thereof in the organic compound layer having a thickness of about 100 to 200 nm is about several tens nanoseconds in light of carrier mobility in the organic compound layer. Luminescence response time, which includes the step from the recombination of the carries to luminescence, is a time in order of microseconds or less. Therefore, the light emitting element also has an advantage that the response thereof is very rapid.
On the basis of such properties, for example, realizability of a thin and light form, rapid responsibility, and capability of being driven with a low DC voltage, attention is paid to the light emitting element as a flat panel display element in the next generation. Moreover, the light emitting element is relatively easy to watch since the light emitting element is of the natural light type and the field angle thereof is wide. Thus, it can be considered that the light emitting element is effective as an element used in a display screen of electrical appliances.
Such a light emitting element can be classified into a passive matrix type (simple matrix type) and an active matrix type, dependently on the driving manner thereof. Attention is paid particularly to the active matrix type since highly minute display based on pixels whose number is over QVGA can be realized.
A light emitting device of the active matrix type, having a light emitting element, has an element configuration as illustrated in FIG. 18. A TFT 1902 is formed on a substrate 1901, and an interlayer insulating film 1903 is formed on the TFT 1902. The interlayer insulating film 1903 can be made of an inorganic material containing silicon, such as silicon oxide or silicon nitride, or an organic material such as an organic resin material (for example, polyimide, polyamide, or polvacrylate). In order to make the surface of the substrate flat, the organic material is more suitable.
On the interlayer insulating film 1903 is formed an anode (pixel electrode) 1905 connected electrically to the TFT 1902 through an wiring 1904. As the material of the anode 1905, a transparent conductive material having a large work function is suitable. As examples thereof, there are suggested: indium tin oxide (ITO), tin oxide (SnO2), an alloy made of indium oxide and zinc oxide (ZnO), a golden semipermeable membrane, polyaniline and the like. Of these, ITO is most frequently used since ITO has a band gap of about 3.75 eV and high transparency against visible light rays.
Examples of the method of forming a film of ITO include chemical vapor deposition, spray pyrolysis, vacuum evaporation, electron beam evaporation, sputtering, ion beam sputtering, ion plating, and ion assist evaporation. In recent years, sputtering has been frequently used in industry.
An organic compound layer 1906 is formed on the anode 1905. (In the specification, all layers disposed between an anode and a cathode are defined as organic compound layers.) Specifically, the organic compound layer 1906 includes one or more of a light emitting layer, a hole injection layer, an electron injection layer, a hole transport layer, an electron transport layer or the like. Basically, the light emitting element has a structure wherein an anode, a light emitting layer and a cathode are successively deposited, or may has, for example, a structure wherein an anode, a hole injection layer, a light emitting layer and a cathode are successively deposited, or a structure wherein an anode, a hole injection layer, a light emitting layer, an electron transport layer and a cathode are successively deposited.
Examples of the method of forming a film of the organic compound making into the organic compound layer 1906 include vapor deposition, printing, inkjet printing, and spin coating. The vapor deposition, which makes application-sharing (i.e., coating-sharing) possible by use of metal masks, is frequently used to make a film of a low molecular weight organic material.
After the organic compound layer 1906 is formed, a cathode 1907 is formed. In this way, a light emitting element 1908 is formed. In FIG. 18, only the light emitting element formed in one pixel is illustrated. Actually, however, a plurality of the light emitting elements are formed in a pixel section so as to form a light emitting device of an active matrix type.
For the production of a light emitting device, an improvement in its electrodes is important in order to make the device characteristic higher.
However, several problems remain about the formation of the anode. In the case of an active matrix type element configuration as described above, two problems as stated below are caused since its anode is formed to contact its interlayer insulating film.
The one is a problem caused from the fact that the temperature characteristic of the organic resin material which makes into the interlayer insulating film is different from that of the transparent conductive film (ITO) constituting the anode. Specifically, the thermal expansion coefficients, depending on temperature, of the two materials formed to contact each other are different from each other: therefore, cracks are easily generated near the interface between the two materials and inside the material having a smaller thermal expansion coefficient when heat is supplied to the two materials.
FIG. 2A shows relationship between temperature and thermal expansion coefficient. Its transverse axis is taken along temperature, and its vertical axis is taken along thermal expansion coefficient. Lines 201 and 202 represent the thermal expansion coefficient of the organic resin material (PI: polyimide) which makes into the interlayer insulating film and that of the transparent conductive film (ITO) constituting the anode, respectively. When temperature is Tx in this graph, the thermal expansion coefficient of the organic resin material (PI) is a1 and that of ITO is a2.
The following relationship is established about these thermal expansion coefficients: a1>a2. Therefore, in the case that an organic resin material 212 and an ITO 213 are formed on a substrate 211 so as to overlap with each other as illustrated in FIG. 2B, cracks 214 are generated near the interface between the organic resin material 212 and the ITO 213 and inside the ITO 213, as illustrated in FIG. 2B.
The ITO is an anode of the light emitting element, and is an electrode for injecting holes related to luminescence. Therefore, when the cracks are generated in the ITO, the generation of the holes is influenced or the holes to be injected are reduced. Moreover, the light emitting element deteriorates.
The other is a problem about gas generated from the interlayer insulating film made of the organic resin material such as polyimide, polyamide, or polyacrylate. It is known that in general a light emitting element deteriorates easily by oxygen or water. Thus, the deterioration of the light emitting element is promoted by gas, such as oxygen, generated from the interlayer insulating film.
Furthermore, there is a problem resulting from the flatness of the surface of the anode. This is a problem common to both of the passive matrix and active matrix types. If the flatness of the anode surface is poor, the thickness of the organic compound layer formed on the anode becomes uneven. When the thickness of the organic compound layer in the light emitting element is uneven in this way, an electric field is unevenly applied thereto so that electric density in the organic compound layer also becomes uneven. As a result, the brightness of the light emitting element drops, and further the life span of the light emitting element becomes short because of acceleration of the deterioration of the element.